Oil shale is found throughout the world and would constitute a plentiful and relatively inexpensive fuel if techniques were available for quickly and inexpensively processing the oil shale into combustible gases. One approach to processing oil shale into combustible gases is disclosed in U.S. Pat. No. 4,211,606 (the disclosure which is hereby incorporated by reference). In this patent, oil shale is heated in a dryer using clean, hot flue gases producing heated shale that is applied to a pyrolyzer. The heated shale is contacted and further heated in the pyrolyzer with ash from the hot flue gases to produce combustion products, and carbonaceous material that is added to a gasifier. Hot gases, ash, and steam are applied to the gasifier such that the carbonaceous material produces further combustible gases. The residue of the gasifier is extracted and applied to what the patent terms an air jet furnace, details of which are disclosed in U.S. Pat. No. 4,110,064 which is also incorporated by reference.
The air jet furnace produces products in the form of hot flue gases whose major constituent is carbon dioxide, and ash particulate which is applied to a separator which separates the products into a stream of hot gases and comparatively coarse ash, and a stream of hot gas and comparatively fine ash. A portion of the comparatively fine ash is applied to the gasifier and the stream of hot gases and fine ash are applied to a further separator that produces the clean flue gases that serve to heat the shale in the dryer.
A derivative of the apparatus described above has apparently been used in two plants in the U.S.S.R. in 1990 and 1991. As presently understood, the actual design eliminates the gasifier. Oil shale is fed into a pyrolyzer wherein pyrolyzation takes place producing combustible products which are extracted, and carbonaceous material after a predetermined residence time of the shale in the pyrolyzer. This material is supplied to an air jet furnace wherein combustion takes place producing hot flue gases, and particulate that is applied to a separator which separates the flow into two streams, one of which contains comparatively coarse ash, and the other of which contains flue gases and comparatively fine ash. The comparatively coarse ash is applied to the pyrolyzer which produces pyrolysis gas at a temperature in excess of 400.degree. C. Such gas contains combustible products, steam, and carbon compounds. The stream containing the comparatively fine ash and flue gases is applied to another separator producing relatively clean flue gases which nevertheless still contain a relatively large quantity of ash. These gases so produced together with the pyrolysis gas are both applied to a burner that is a part of the combustion chamber of a boiler where combustion takes place, the boiler producing steam that may be used for generating electricity.
Even when the pyrolyzer is constructed as a rotating drum, the residence time for the shale in the pyrolyzer to become completely pyrolyzed is relatively long which results in excessively large physical size, great complexity in the equipment, and high cost. It is therefore an object of the present invention to provide a new and improved design for producing combustible gases from low grade solid fuel which reduces the size of the pyrolyzer by reducing the residence time of the low grade fuel therein.